Can bridging two networks in Windows Server 2012 lead to an external attack? I have two LAN networks on my server. I tried to bridge them, but the server got attacked post bridging. Does bridging open additional ports on the existing connections, and can bridging alter the networking system for other systems connected on the network? I am not a networking guy so I am not sure if this is possible or not.

You should not bridge links with different network addresses, this can cause problems. Bridging is the same thing as connecting the links on the same VLAN with a switch. Layer-2 traffic is transparently passed through the bridge.

If you were attacked through one of the links, you had those links anyway. Your attack could come through any link attached to your system.